In the screening of wood chips, a high speed of the screening movement is aimed at, out of two reasons:
in order to increase the screening capacity, and
in order to keep the screen planes open.
The remaining of the screen planes open is made more difficult by wood pieces and splinters that get stuck in the holes, as well as by sawdust that contains resin, moisture, snow, etc., which blocks the screen planes with the smallest holes and is deposited on the inner faces of the screen. By means of a sufficiently rapid screening movement, the said drawbacks can be reduced to a considerable extent, which increases the capacity of the screen indirectly further.
Another aim is a constant increase in the size of the screen.
In the screening of chips, the screening movement usually takes place in the horizontal plane, while the screen planes are slightly forwardly slanting. Almost without exception, the present-day chip screens are provided with counterweights,
either in order to counterbalance the screening movement, or
in order to produce the screening movement, by rotating the counterweights.
The movement needed for screening is produced by means of a vertical machine member (shaft, crankshaft or equivalent) revolving in the screen basket.
The highest speed of the screening movement is, as a rule, limited by the mass forces acting upon the bearings of the machine member concerned. Some of these forces are unavoidable (e.g. horizontal forces of the screen basket), some are not (e.g. horizontal and vertical forces of the counterweight).